Need Help Setting Up G27 Clutch

Is there some setting in NR2003 so you have to press in the clutch, like you can with GPLShift, in order to shift gears? I'm just looking for that extra touch of realism, although I don't know why I need the extra torture.

There is no setting that is going to force you to engage a clutch in order to change gears. Even in a passenger, you can choose to change gears without engaging the clutch; you may damage your gearbox (if you do) but you can still do it.

When running cup, gns, and cts in NR2003, if you don't lift off the throttle when changing gears, eventually you'll break the car. The reason given may vary, e.g. gearbox, clutch, header, etc., but the car will cease to work. However, there is no need to ever engage a clutch, either manually or via "Options/ Controls/ Driving Aids/ Automatic Clutch". The pta and gtp mods don't require any throttle lift when changing gears.

If you want to use a clutch pedal, just setup your device (G27) so your clutch pedal is mapped to the clutch action, "Options/ Controls/ Control Assignments - Clutch".
[Depending on the way your Logitech G27 software is used/set up, you may or may not have to map the device actions in the Logitech Profiler.]

My advice was given on the background of the GTP mod.
If you disable Autoclutch then the engine/gearbox will break after 1-10 downshifts. Dependent of how harsh your shifting is.
Upshifts works mostly OK (if you are gentle:-)

When running cup, gns, and cts in NR2003, if you don't lift off the throttle when changing gears, eventually you'll break the car. The reason given may vary, e.g. gearbox, clutch, header, etc., but the car will cease to work. However, there is no need to ever engage a clutch, either manually or via "Options/ Controls/ Driving Aids/ Automatic Clutch". The pta and gtp mods don't require any throttle lift when changing gears.

I thought I was clear on the use of the clutch in all the series/mods, including pta and gtp, it's not required. Throttle lift is needed for the stock cars but not pta and gtp.
I don't use a clutch or the "auto-clutch" aid for any series/mods. I don't lift off the throttle when I change gears in pta and gtp. I don't experience any failures for 100's (or perhaps 1000's) of gear changes. (The only time I ever lose a gear is in the pitbox before the start of a qualifying run.)